(1) Field of the Invention
The present invention relates to an optical element, and in particular to an optical element wherein optical waveguides formed on different faces of the same substrate are optically connected. In addition, it relates to an optical element for applying the same modulating signal into different optical waveguides.
(2) Related Art Statement
In recent years, external optical modulators such as an optical waveguide device have been widely used in the field of optical communication or the like for the purpose of enabling high-speed switching. Optical waveguide modulators consisting of lithium niobate (LiNbO3; hereinafter referred to as LN), which have features of high frequency characteristic, low insertion loss, high extinction ratio or the like, and optical waveguides formed by thermal diffusion of Ti on said substrate, have been put to practical use.
In improving or adding functions of the optical element such as an optical modulator, the interaction length of an optical waveguide and a modulating electrode for applying electric field into said optical waveguide is designed to be long, or different electrodes having each function are provided, as the conventional ways. For example, a method for lengthening said interaction length for decreasing driving voltage, or a method for separating electrodes for a high frequency electrical signal and for DC bias in view of convenience of modulation control can be cited.
In addition, an optical intensity modulator, phase modulator, and polarization modulator can be cited as the optical modulator. Some of these modulators are combined to form various optical elements.
These cause the optical modulator to grow in size, generating problems that the production costs get higher and that the size of the device using the optical modulator is limited. Moreover, when several electrodes are placed on a limited size substrate, it is not possible to set the electrodes of enough length in relation to the arrangement of electrode.
On the other hand, the following patent document 1 proposes that several optical modulated portions are formed on parallel in the same face of a substrate and a light wave is turned back at the side edge of the substrate to thereby prevent the substrate from increasing in length. However, if several optical modulated portions are placed in parallel as in the patent document 1, the width of the substrate gets larger. The problems that the production costs get higher and the size of the device using the optical modulator is limited still remain unsolved.
[Patent Document 1] Japanese Patent Application Publication No. 2001-350046
In addition, as the example of the optical element, an optical waveguide 102 for guiding a light wave, and a modulating electrode 103 and ground electrodes 104 for applying a electrical high-speed modulating signal into said light wave are formed on a substrate 101 having an electrooptic effect as shown in FIG. 12. FIG. 12(b) is the cross-section view of FIG. 12(a) along the chain line A. Although it is omitted in FIG. 12(a), a buffer layer such as SiO2 is formed between the substrate and the electrode. Optical waveguide 102 is a Mach-Zehnder type optical waveguide (hereinafter referred to as a MZ type optical waveguide) comprising two branching optical waveguides, whereby the light wave being propagated through said optical waveguide receives optical modulation by single modulating electrode 103.
The optical element as in FIG. 12 has high driving voltage related to the optical modulation, and the electric fields applied into two branching optical waveguides are not symmetric. Thus, chirping is generated in the modulated light wave from the optical element, and this could be a cause of limiting distance of transmission. Therefore, for the purpose of lower driving voltage and zero chirp, the optical element comprising two modulating electrodes 105 as shown in FIG. 13, hereinafter referred to as a dual-type element, has been put to practical use. However, it is necessary for the dual type element as shown in FIG. 13 to apply the same amplitude and anti-phase signal into both of modulating electrodes. This setup could cause higher costs since adjustment of the electrical signals gets complicated and the number of components increases.
The patent document 2 discloses that ferroelectric spontaneous polarization (FSP) of one part of a substrate, especially a region of the substrate comprising one branching optical waveguide is reversed, and two modulating electrodes needed in FIG. 13 are composed of modulating electrodes having branching lines that branch from a single line. The optical element utilizing such FSP reversal realizes low driving voltage and zero chirp likewise the dual type element while the setup to apply a modulating signal into the modulating electrodes can be simplified as well as the single modulating electrode as in FIG. 12.
[Patent Document 2] Japanese Patent Application Publication No. 2003-202530
At the same time, the patent document 3 proposes that an optical waveguide formed on a substrate having an electrooptic effect is turned back at one side edge of the substrate in order to shorten the optical element and realize polarization-independency. As shown in FIG. 14, an optical waveguide 110 formed on a substrate 101 is turned back at one side edge (right side of the figure) of the substrate, and modulating electrodes 113 and 115, and ground electrodes 114 and 116 are formed on each branching optical waveguide. The polarization-independent element controlled light locates a reflecting mirror 112 and a ¼ wavelength plate 111 at the turnback so as to rotate the plane of polarization of the propagated light wave by 90 degrees.
[Patent Document 3] Japanese Patent Application Publication No. H07-325276
It is necessary for the optical element having such turnback optical waveguide to use a single modulating electrode 120, for example, as shown in FIG. 15 in order to apply a modulating signal into modulating electrodes 113 and 115. This could cause the problems that driving voltage increases and that chirping is generated as in FIG. 12. 121 and 122 in FIG. 12 show the ground electrodes. If two modulating electrodes as shown in FIG. 13 are used, it becomes necessary to use a jumper lead because signal lines are crossed when connecting electrodes 113 and 115, and electrodes 114 and 116 respectively. This jumper lead could be a cause that propagation loss of the modulating signal dramatically increases.
The present invention intends to solve the above described problems, to provide a highly-integrated and compact optical element, and further, to provide an optical element having various functions such as lower driving voltage, chirping suppression, and polarization-independency.
Moreover, the present invention intends to provide wiring capability of the modulating electrode in the optical element even having the complicated optical waveguides such as the optical waveguide turned back at one side edge of the substrate, and further, capable of realizing low driving voltage and zero chirp.